Method and device for calibrating a humidity sensor and sensor arrangement comprising a humidity that can be calibrated

ABSTRACT

For calibrating a moisture sensor ( 1 ), the latter is used to measure the relative moisture (U1, U2) at each of two known gas pressure values (P1, P2), likewise to be measured if necessary. At the same time, other ambient conditions are kept constant. A correction value for correction of the measured values of the moisture sensor to be subtracted from the latter is then obtained from the formula: k=((P1/P2)*U2−U1)/(P1/P2−1). The moisture sensor ( 1 ) is thus capable of calibration without measurement of the actual moisture value.

BACKGROUND INFORMATION

[0001] The present invention relates to a method of calibrating a moisture sensor, in which a moisture value is in each instance detected by the moisture sensor under first ambient conditions and under second ambient conditions different from the first, and in which a correction value for the moisture sensor is determined from the parameters of the ambient conditions and the measured moisture values.

[0002] Moisture sensors are used to measure the relative moisture in open or closed volumes of gas, i.e., the quantity of moisture that is dissolved in the particular gas relative to the maximum quantity of moisture soluble in the gas at the given temperature.

[0003] Such measurements are familiar to, for example, the average consumer as measurements of relative atmospheric moisture for the determination of climatic conditions, but they are also common in the industrial field for determination of, for example, the relative atmospheric moisture in gas vessels under pressure.

[0004] Moisture sensors are often severely stressed by physical and chemical influences and their response characteristic may thereby be significantly altered with respect to the initial calibration. Recalibration is therefore necessary at certain intervals.

[0005] The related art (German Patent Application 3936138 A1, U.S. Pat. No. 6,073,480) includes calibration of a moisture sensor in that a first moisture measurement is carried out at a first temperature and then a second moisture measurement is carried out at a second temperature, the gas pressure remaining the same in both measurements. The measured temperature and moisture values permit computation of a correction value for the moisture measurement, since the mathematical relationship between temperature and actual relative moisture is known.

[0006] However, for the use of this technique it is necessary to place temperature sensors in the region of the moisture sensors, as well as to provide heating or cooling elements. In addition, such a method of calibration requires time for setting the particular measurement temperatures.

[0007] In contrast to this, the object of the present invention is to provide a method of calibrating a moisture sensor and a corresponding device and sensor arrangement having a calibratable moisture sensor of the type mentioned at the beginning, wherein the calibration procedure is simplified and as few as possible auxiliary means are required for its performance.

[0008] According to the present invention, this object is achieved in that, at a first pressure P1 and at a second pressure P2 different from the first and at otherwise like conditions, a moisture value U1, U2 is detected in each instance, at least the ratio of first pressure value P1 and second pressure value P2 being known, and in that the correction value for the moisture sensor is determined from the ratio of the pressure values and the measured moisture values.

[0009] The object is in addition achieved by a device having a pressure sensor, an input means for input of the measured values of the moisture sensor into a first data memory and a computing means that determines a correction value of the moisture sensor from the ratio of two pressure values and two moisture values detected at the particular pressure values under otherwise like conditions.

[0010] If the moisture value is measured at two unlike pressures, the temperature and other ambient conditions being kept constant and the same quantity of absolute moisture remaining dissolved in the gas, the value of the relative moisture between the two measurements varies. The ratio of the two actually existing moisture values of the relative gas moisture may be inferred from the ratio of the two pressure values. If the ratios of the pressures and the measured relative gas moistures do not agree, the correction value that is to be applied to the particular moisture measurement may be determined from the difference. This correction value is then to be subtracted from or added to the measured value of the relative moisture.

[0011] The device for calibrating a moisture sensor accordingly has a pressure sensor, if the latter is not already provided in the system in the region of the moisture sensor, as well as an input means via which the measured moisture values of the moisture sensor may be supplied to the calibrating device. A computing means of the calibrating device then determines the correction value from the measured data and outputs it.

[0012] The method of calibration advantageously is carried out in that correction factor k to be subtracted from a measured moisture value is determined according to the equation:

k=((P1/P2)*U2−U2)/(P1/P2−1).

[0013] Ideally, for a gas of constant consistency and constant temperature, the ratio of the two pressure values at which the measurement is made corresponds to the ratio of the actually existing relative air moistures:

P1/P2=U1 (actual)/U2 (actual).

[0014] Since the measured moisture values U1, U2 do not agree with the actual moisture values before calibration, the actual moisture value

U1 l (actual)=U1−k and U2 (actual)=U2−k

[0015] is computed.

[0016] The equation:

P1/P2=(U1−k)/(U2−k)

[0017] is obtained.

[0018] Solved for k, this equation gives:

k=((P1/P2)*U2−U1)/(P1/P2−1).

[0019] This applies on the assumption that k is independent of the value of U. The calibration method according to the present invention thus permits the moisture sensor to be calibrated with the use of a pressure sensor without the actually existing moisture being directly determined. It is only necessary to use a calibrated pressure sensor, which, however, is simple in that pressure sensors are relatively stable in their calibration.

[0020] In practice, the method of calibrating a moisture sensor in a pressurizable gas-filled system is advantageously carried out in that gas is discharged or supplied by a valve provided in the system, and in that pressure values and moisture values are recorded before and after the discharge/supply of gas, temperature equalization being awaited before each moisture measurement.

[0021] Pressurizable gas-filled systems typically are already provided with valves for filling with gas or for discharging gas. Gas may be discharged or supplied via such a valve, and discharged gas may be stored in an external pressure tank. Moisture measurements are made each time before and after discharge and supply of gas, and temperature equalization must be awaited after discharge or supply of gas, since both moisture measurements must take place at the same temperature and since, owing to expansion of the gas and owing to pressure increase, a temperature reduction or increase is to be awaited. Instead of valves, pressure reducers may alternatively be used.

[0022] For calibrating the moisture sensor, the latter may be operated in a small, pressure-tight closed part (measuring chamber) of the system, so that the different pressure values do not require a pressure modification in the entire system, but only the gas pressure in the partial volume in which the moisture sensor is located need be varied. The corresponding pressure sensors may already be provided in the system in the region of the moisture sensor, but they may alternatively also be introduced into the volume in the course of calibration or the particular pressure may be measured outside the system upon discharge or supply of gas in the respective feed line. Hence calibration may be carried out simply, reliably and rapidly by service personnel, with no major changes or installations in the pressurizable gas-filled system. The measuring chamber having a built-in moisture sensor is thus designed to produce, by suitable means (valve, pressure reducer, etc.), the two pressures for the moisture sensor required for the calibration method.

[0023] The present invention in addition relates to a sensor arrangement having a moisture sensor capable of calibration according to the method as recited in one of claims 1 through 3 and having a correction means with a memory, in which a correction value to be subtracted from the measured moisture value is storable, the correction means subtracting the correction value from the detected measured value of the moisture and in particular delivering the result to a display means.

[0024] The sensor arrangement thus has a correction means 8 by which, after a calibration, the moisture value detected by the moisture sensor is corrected by the correction value, so that the actually existing moisture value is available as a corrected measured value for reading off or for further processing in, for example, a control room.

[0025] The present invention is shown below by way of example in the drawing and is then explained.

[0026]FIG. 1 in a schematic representation shows a moisture sensor and a device for its calibration,

[0027]FIG. 2 schematically shows the function of the computing means for determination of the correction value,

[0028]FIG. 3 shows a flow diagram of the calibration procedure.

[0029]FIG. 1 shows a moisture sensor 1 in a gas-tight measuring chamber 2. Measuring chamber 2 has a first gas connection 3 and a second gas connection 4, by which the measuring chamber may be connected to a gas-filled pressurizable system 5 and to an additional gas chamber by a pressure hose 6.

[0030] A pressure sensor 7 is additionally provided in measuring chamber 2. This pressure sensor may be a conventional pressure sensor, but it should be calibrated and have a measurement accuracy that is as high as possible.

[0031] In order to measure, for example, the gas moisture in pressurizable system 5 and at the same time calibrate moisture sensor 1, measuring chamber 2 is first connected via first gas connection 3 to pressurizable system 5, so that gas exchange and pressure equalization may take place. Then the gas pressure in the measuring chamber is measured by pressure sensor 7 and measuring chamber 2 is separated from the pressurizable system. In addition, under the conditions now existing, the first moisture value is measured by moisture sensor 1. At the same time, correction means 8, which is connected to feed lines 9 of the moisture sensor, and which in normal operation subtracts the correction value stored in memory 10 from the direct measured value of moisture sensor 1, is bypassed for calibration.

[0032] A fixed quantity of gas is then discharged from measuring chamber 2 by second gas connection 4 via pressure hose 6, so that the gas pressure is reduced. Then measuring chamber 2 is again closed gas-tight.

[0033] After the temperature in measuring chamber 2 has returned to normal, a second moisture value U2 and a second pressure value P2 are measured. From the two measured pressure values and the two measured and uncorrected moisture measurement values a correction value may then be determined, which is stored in memory 10. On subsequent moisture measurements within correction means 8 the correction value stored in the memory is then subtracted from the moisture value measured by moisture sensor 1 in a subtraction unit 11 and output as the corrected measured value by a display 17.

[0034] The detection and computation steps in a calibration procedure are represented schematically in FIG. 2. In a first step, moisture value U1 is detected by moisture sensor 1. At the same time, pressure value P1 is detected by pressure sensor 7. The two measured values are stored in a first data memory 12 and a second data memory 13.

[0035] In a second step, at reduced pressure and otherwise like ambient conditions, a second moisture value U2 and a second pressure value P2 are measured. The two measured values are again stored in corresponding first and second data memories 12, 13.

[0036] Then the P1/P2 ratio is computed and stored in a third data memory 14. In a computing means 15, k is computed by a microprocessor according to the formula given in claim 2 and sent on to an output means 16, which sends the correction value on to, for example, correction means 8, where it is stored in memory 10.

[0037] In FIG. 3 the calibration procedure is again represented in the form of sequential steps. In a first step 101, a specified pressure is set in the measuring chamber. In second step 102, pressure value P1 and moisture value U1 are measured. In third step 103, the pressure in the measuring chamber is varied by, for example, discharging gas. In fourth step 104, a second pressure P2 and a second moisture value U2 are measured. In fifth step 105, the quotient of P1 and P2 and the quotient of U1 and U2 are computed. In sixth step 106, the quotients of P1 and P2 and of U1 and U2 are compared with one another. If the two quotients agree, in the next step (109) the correction value k=0 is set and the calibration procedure ended. If the quotients fail to agree, in next step 107 k is computed from the formula given in claim 2. In eighth step 108, k is then stored in memory 10 for correction of subsequent moisture measurements.

[0038] In an exemplary calibration, a first pressure value P1=4.5 bar was measured initially. At this pressure value a moisture value U1=1.7% relative moisture was measured by the moisture sensor. At a second gas pressure, pressure value P2=0.9 bar and a moisture value of 0.5% relative moisture were measured. 5 was obtained as the quotient of the pressure values. Hence the relative moisture value at a high pressure of 1.7% relative moisture would have to drop to 0.34% relative moisture at the lower pressure value. However, a value of 0.5% relative moisture was measured at the lower pressure value. In other words, a correction is necessary, k being obtained as follows:

k=(5*0.5% rel. moist.−1, 7% rel. moist.)/(5−1)=0.2% rel. moist.

[0039] Then the corrected moisture values according to the measurement carried out are U1 (actual)=1.7% rel. moist−0.2% rel. moist.=1.5% rel. moist. and

U2 (actual)=0.5% rel. moist.−0.2% rel. moist.=0.3% rel. moist.

[0040] The calibration method described may be performed cyclically or periodically during operation of the moisture sensor and a new correction value computed and stored each time.

[0041] The method according to the present invention may be implemented in a variety of ways in addition to the embodiments shown. In particular, there are numerous embodiments of devices for performing the method according to the present invention. 

1. A method of calibrating a moisture sensor, in which a moisture value is in each instance detected by the moisture sensor under first ambient conditions and under second ambient conditions different from the first, and in which a correction value for the moisture sensor is determined from the parameters of the ambient conditions and the measured moisture values, wherein, at a first pressure and at a second pressure different from the first and at otherwise like conditions, a moisture value is detected in each instance, at least the ratio of the first pressure value and the second pressure value being known; and the correction value for the moisture sensor is determined from the ratio of the pressure values and the measured moisture values.
 2. The method as recited in claim 1, wherein a correction factor k to be subtracted from a measured moisture value is determined according to the equation: k=((P1/P2)*U2−U1)/(P1/P2−1).
 3. The method as recited in claim 1, wherein gas is discharged or supplied by a valve provided in the system; and pressure values and moisture values are recorded before and after the discharge/supply of gas, temperature equalization being awaited before each moisture measurement.
 4. A device for calibrating a moisture sensor, comprising: a pressure sensor, an input means for input of the measured values of the moisture sensor into a first data memory, and computing means that determines a correction value of the moisture sensor from the ratio of two pressure values and two moisture values detected at the particular pressure values under otherwise like conditions.
 5. A sensor arrangement, comprising: a moisture sensor, and correction means with a memory, in which a correction value to be subtracted from the measured moisture value is storable, the correction means subtracting the correction value from the detected measured value of the moisture and delivering the result to a display means.
 6. The method as recited in claim 2, wherein gas is discharged or supplied by a valve provided in the system; and pressure values and moisture values are recorded before and after the discharge/supply of gas, temperature equalization being awaited before each moisture measurement.
 7. A method of calibrating a moisture sensor, comprising: measuring a first pressure; measuring a first moisture value; measuring a second pressure; measuring a second moisture value; determining a correction value that varies according to the first and second pressures and the first and second moisture values; and maintaining the correction value to be applied to moisture measurements of the moisture sensor.
 8. The method of claim 7, wherein determining the correction value includes using the equation: k=((P1/P2)*U2−U1)/(P1/P2−1), where k is the correction value, P1 is the first pressure, P2 is the second pressure, U1 is the first moisture value, and U2 is the second moisture value.
 9. The method of claim 7, wherein the first moisture value and the first pressure are measured at a first time and wherein the second moisture value and second pressure are measured at a second time different from the first time.
 10. The method of claim 7, wherein the second pressure is measured after the first pressure is measured and wherein the second pressure is less than the first pressure.
 11. The method of claim 7, further comprising: storing the correction value in a memory location that is accessed when the correction value is being applied to moisture measurements of the moisture sensor.
 12. The method of claim 7, wherein the correction value is subtracted from moisture measurements of the moisture sensor.
 13. A moisture sensor, comprising: a memory containing a correction value that varies according to first and second pressure measurements and first and second moisture value measurements; and means for applying the correction value to measurements of the moisture sensor.
 14. The moisture sensor of claim 13, wherein the correction value varies according to the equation: k=((P1/P2)*U2−U1)/(P1/P2−1), where k is the correction value, P1 is the first pressure, P2 is the second pressure, U1 is the first moisture value, and U2 is the second moisture value.
 15. The moisture sensor of claim 13, wherein the means for applying includes means for subtracting the correction value from moisture measurements of the moisture sensor. 